Queen Berúthiel’s cats is one of only two references in The Lord of the Rings that had no pre-written backstory or existence outside of the book, according to a letter that Tolkien wrote in 1956.
However, the story of the Queen and her cats was finally given in Unfinished Tales, published by Christopher Tolkien in 1980. The story tells how Queen Berúthiel of Gondor used ten cats as her spies. She was feared and hated by the people, and finally the king banished her. She boarded a ship with all her cats, and sailed out into the southern seas never to be seen again.